In such machines the rotatable yarn supply means supplies yarn under essentially slipless conditions to the textile machine. Yarn guide elements are associated with the yarn supply means, and a speed controlled motor is coupled to and controls the rotation of the yarn supply means. A movable yarn tension element is positioned in the path of the yarn from the yarn supply means to the textile machine, and yarn tension bias force means are coupled to the yarn tensioning element for applying predetermined, controllable tension bias force to the yarn. A yarn reserve zone formed in the path of the yarn from the yarn tension element to one of the yarn guide elements is positioned downstream, in the path of the yarn to the textile machine, from the yarn tension element, for storing a quantity of reserve yarn as a function of the yarn tension element position. Electrical control means are coupled to the motor and control the speed and rotation of the motor as a function of the yarn required by the textile machine.
In flat knitting machines, for example, the carriage that carries the needle cam portions and with them controls the needle movement executes a reciprocating movement over the effective length of the needle bed. In order to insert the yarn into the hooks of the projected needles properly, the yarn guide must also be reciprocated correspondingly. In the vicinity of its motion reversal points, the carriage executes an excess stroke with respect to the needles most recently involved in knitting, and during this excess stroke no yarn is processed and accordingly no yarn is unreeled from the yarn source. Furthermore, the length of the yarn travel path between the stationary yarn source and the yarn guide, which executes a linear reciprocating movement, undergoes constant change. To prevent intermittently excessive yarn material from forming a loop, a so-called yarn tensioner is used in practice, which receives the yarn not required in a yarn reserve and keeps it under tension until such time as the reversal movement of the carriage and yarn guide is completed and the first needle is again performing knitting, or in other words the yarn reserve formed is used up as the yarn travel path to the yarn guide again increases in length as movement continues. Fundamentally similar conditions arise when socks and stockings with heels and toes are knitted on small circular knitting machines, if the needle cylinder is moved back and forth in the circumferential direction, in so-called shuttle motion, to knit the heel or toe.
These yarn tensioners operate with yarn brakes and have the basic disadvantage that they cannot assure yarn tension that remains uniform. As a result, the loop size is variable, so that socks and stockings produced in this way, for instance, are of different lengths, and accordingly have to be sorted into pairs of equal length after they have been completed.
Also operating with a yarn brake is a yarn retrieval apparatus, known from French Publication Document FR-OS No. 2538 419, for flat knitting machines or for stocking or sock knitting machines operating in shuttle motion, in which the yarn tension along the yarn travel path between a stationary yarn guide element and the yarn guide is sensed or scanned, and a yarn brake and retrieval device disposed upstream of the stationary yarn guide element in the yarn travel path to the knitting machine is adjusted in such a way that the yarn tension at the yarn guide remains approximately within a predetermined fluctuation range. Aside from the fact that with this device, operating with limit switches controlled by the yarn sensing element, only a very unsatisfactory, crude variation of the yarn tension is possible, the yarn must also be drawn from the spool by the needles them-selves, via the yarn brake, so that the yarn can be processed only with a relatively high yarn tension. Furthermore, disruptions in the unreeling of the yarn from the spool can deleteriously affect the uniformity of the knitted goods.
This disadvantage is overcome in a yarn supply apparatus for a flat knitting machine known from U.S. Pat. No. 3,962,891, on which the present invention is based, by means of the provision of a rotatably supported yarn supply element located downstream in the yarn travel path of the yarn spool that supplies the yarn in a slip-free manner. This yarn supply element is driven by an electric drive motor and supplies the yarn to the yarn guide and hence to the needles. The arrangement is such that travel transducers are connected to the drive elements of the carriage and yarn guide of the flat knitting machine, emitting electrical signals representative of the particular position and speed of the carriage and yarn guide; with the aid of these signals the drive motor of the yarn supply element is controlled as a function of the yarn usage over time, taking into account the changes in the yarn travel path to the yarn guide that occur during the reciprocating movement of the yarn guide. To this end, the drive motor, as a final control element, is located in an electrical control loop, the reference variable of which is formed by the aforementioned signals. To prevent fluctuations in the yarn tension during the acceleration and deceleration phases of the drive motor from occurring during the reversal of motion of the yarn guide, because the inertial mass of the drive motor prevents compensating for such fluctuations by changing the rpm of the yarn supply element, a yarn tensioning element is disposed downstream of the yarn supply element in the yarn travel path, and if the yarn tension decreases the yarn tension element temporarily builds up a yarn reserve, which as operation continues is then used up again. This yarn tensioning element is in the form of a yarn guide arm supported such that it is rotatable or pivotable about a stationary axis of rotation, and at one end it has a yarn eye that in cooperation with stationary yarn guide elements generates an approximately V-shaped yarn travel path. The yarn guide arm is coupled in the vicinity of its support point to a spiral spring anchored in a stationary fashion at one end, which exerts an adjustable, predetermined bias force upon the yarn guide arm; this force determines the magnitude of the yarn tension. Installing the various travel transducers on the flat knitting machine necessitates intervention into the machine. Furthermore, these travel transducers are necessarily expensive, because they detect the entire stroke movement of the carriage and of the yarn guide over the often relatively long needle bed, and moreover must be at least partly adjustable to suit the width of the particular knitted article being made. Since the drive motor of the yarn supply element is controlled only in a rigid predetermined dependency on the reciprocating movement of the carriage and of the yarn guide, the calibration and adjustment of the individual members of the control system are critical.